So far, to handle exceptions, we were using the try,
catch, and throw keywords. These allowed us to perform normal
assignments in a try section and then handle an exception, if any, in
a catch block.

In the previous lesson, we mentioned that, when you
create a stream, the operating system must allocate resources and dedicate
them to the file processing operations. Additional resources may be provided
for the object that is in charge of writing to, or reading from, the stream.
We also saw that, when the streaming was over, we should free the resources
and give them back to the operating system. To do this, we called the
Close() method of the variable that was using resources.

More than any other assignment, file processing is in
prime need of exception handling. As we will see in the next section, during
file processing, there are many things that can go wrong. For this reason,
the creation and/or management of streams should be performed in a try
block to get ready to handle exceptions that would occur. Besides actually
handling exceptions, the C# language provides a special keyword used free
resources. This keyword is finally.

The finally keyword is used to create a section of an
exception. Like catch, a finally block cannot exist by itself. It can be
created following a try section. The formula used would be:

try
{
}
finally
{
}

Based on this, the finally section has a body of
its own, delimited by its curly brackets. Like catch, the finally
section is created after the try section. Unlike catch,
finally never has parentheses and never takes arguments. Unlike catch,
the finally section is always executed. Because the finally
clause always gets executed, you can include any type of code in it but it
is usually appropriate to free the resources that were allocated earlier.
Here is an example:

In the previous lesson as our introduction to file
processing, we behaved as if everything was alright. Unfortunately, file
processing can be very strict in its assignments. Based on this, the .NET
Framework provides various Exception-oriented classes to deal with almost
any type of exception you can think of.

One of the most important aspects of file processing is
the name of the file that will be dealt with. In some cases you can provide
this name to the application or document. In some other cases, you would let
the user specify the name of the path. Regardless of how the name of the
file would be provided to the operating system, when this name is acted
upon, the compiler is asked to work on the file. If the file doesn't exist,
the operation cannot be carried. Furthermore, the compiler would throw an
error. There are many other exceptions that can be thrown as a result of
something going bad during file processing:

FileNotFoundException: The exception thrown when
a file has not been found is of type FileNotFoundException. Here is
an example of handling it:

Error: Could not find file 'C:\Documents and Settings\Administrator\Local
Settings\Application Data\Temporary
Projects\ConsoleApplication1\bin\Release\Members.clc'.
May be the file doesn't exist or you typed it wrong!
Press any key to continue . . .

IOException: As mentioned already, during file
processing, anything could go wrong. If you don't know what caused an error,
you can throw the IOException exception.

Application:
Handling File Processing Exceptions

To throw exceptions, change the file processing methods from the
IceCream.cs file as follows:

In its high level of support for file processing, the
.NET Framework provides the FileInfo class. This class is equipped to
handle all types of file-related operations including creating, copying,
moving, renaming, or deleting a file. FileInfo is based on the
FileSystemInfo class that provides information on characteristics of a
file.

Application:
Introducing File Information

Start a new Console Application named WattsALoan2

To take advantage of the Visual Basic rich library, in the Solution
Explorer, under WattsALoan2, right-click References and click Add
Reference

In the Add Reference dialog box, click the .NET tab and click
Microsoft.VisualBasic

This constructor takes as argument the name of a file or
its complete path. If you provide only the name of the file, the compiler
would consider the same directory of its project. Here is an example:

FileInfo fleMembers = new FileInfo("First.txt");

Alternatively, if you want, you can provide any valid
directory you have access to. In this case, you should provide the complete
path.

File Creation

The FileInfo constructor is mostly meant only to
indicate that you want to use a file, whether it exists already or it would
be created. Based on this, if you execute an application that has only a
FileInfo object created using the constructor as done above, nothing
would happen.

To create a file, you have various alternatives. If you
want to create one without writing anything in it, which implies creating an
empty file, you can call the FileInfo.Create() method. Its syntax is:

public FileStream Create();

This method simply creates an empty file. Here is an
example of calling it:

The FileInfo.Create() method returns a
FileStream object. You can use this returned value to write any type of
value into the file, including text. If you want to create a file that
contains text, an alternative is to call the FileInfo.CreateText()
method. Its syntax is:

public StreamWriter CreateText();

This method directly returns a StreamWriter
object. You can use this returned object to write text to the file.

File Existence

When you call the FileInfo.Create() or the
FileInfo.CreateText() method, if the file passed as argument, or as the
file in the path of the argument, exists already, it would be deleted and a
new one would be created with the same name. This can cause the right file
to be deleted. Therefore, before creating a file, you may need to check
whether it exists already. To do this, you can check the value of the
Boolean FileInfo.Exists property. This property holds a true
value if the file exists already and it holds a false value if the
file doesn't exist or it doesn't exist in the path.

As mentioned earlier, the FileInfo.Create()
method returns a FileStream object. You can use this to specify the
type of operation that would be allowed on the file.

To write normal text to a file, you can first call the
FileInfo.CreateText() method. This method returns a StreamWriter
object. The StreamWriter class is based on the TextWriter
class that is equipped with the Write() and the WriteLine()
methods used to write values to a file. The Write() method writes
text on a line and keeps the caret on the same line. The WriteLine()
method writes a line of text and moves the caret to the next line.

After writing to a file, you should close the
StreamWriter object to free the resources it was using during its
operation(s).

Application: Writing to a File

To allow the user to create a new employee, change the file as
follows:

Do you want to hire a new employee(0=No/1=Yes)? 1
Hiring New Employee
Enter Employee Number as 00-000: 44-228
Enter Employee Name: Johnny Olney

Close the DOS window

Appending to a File

You may have created a text-based file and written to
it. If you open such a file and find out that a piece of information is
missing, you can add that information to the end of the file. To do this,
you can call the FileInfo.AppenText() method. Its syntax is:

public StreamWriter AppendText();

When calling this method, you can retrieve the
StreamWriter object that it returns, then use that object to add new
information to the file.

Application: Appending to a File

To allow the user to add more employees to the file that holds their
names, change the file as follows: